1. Field of the Invention
The present invention relates to a polarization mode dispersion compensating device, which is a technology applied to an optical transmission system for high speed optical communications using optical fibers, optical switching, optical information processing, etc., and which is particularly useful in compensating the polarization mode dispersion.
The present invention also relates to a technique for effectively compensating the polarization mode dispersion even when a difference between the optical powers of the TE polarization component and the TM polarization component is large.
2. Description of the Related Art
In conjunction with the increase of the transmission capacity due to the advance of the IT field, the bit rate of the optical signals has a tendency of increasing from 2.5 Gb/s to 10 Gb/s, and further to 40 Gb/s. Here, the polarization mode dispersion poses a problem. FIG. 1 is for explaining the polarization mode dispersion, and showing an optical fiber 1001, an input optical pulse 1002, a TE or TM polarization component 1003 of the input optical pulse 1002, a TM or TE polarization component 1004 of the input optical pulse 1002, an output optical pulse 1007, a TE or TM polarization component 1005 of the output optical pulse 1007, and a TM or TE polarization component 1006 of the output optical pulse 1007.
In general, the optical fiber has the polarization mode dispersion due to its birefringence, so that the optical signals propagating through the optical fiber will be propagated by being divided into a fast propagation component and a slow propagation component depending on the polarization planes. In FIG. 1, the polarization component 1003 is the fast propagation component, which becomes the polarization component 1005 at the output end. On the other hand, the polarization component 1004 is the slow propagation component, which becomes the polarization component 1006 that arrives later than the polarization component 1005 at the output end. The output optical pulse 1007 is a sum of the polarization component 1005 and the polarization component 1006, so that the waveform of the output optical pulse 1007 will be distorted as a result.
The amount of the polarization mode dispersion is about 0.2xc3x97L1/2 (ps) to 2xc3x97L1/2 (ps) for a fiber length of L (Km), for example. Namely, assuming the optical fiber of 100 Km long, the polarization mode dispersion of 20 ps can occur at worst. This value is not a serious problem for 2.5 Gb/s (pulse width of 400 ps) or 10 Gb/s (pulse width of 100 ps), but it can cause a fatal waveform distortion for 40 Gb/s (pulse width 25 ps), which in turn can degrade the bit error rate largely.
In order to resolve this problem, conventionally, the polarization mode dispersion has been compensated by a configuration as shown in FIG. 2, which has an input optical fiber 1101, an input optical pulse 1102, a TE or TM polarization component 1103 of the input optical pulse 1102, a TM or TE polarization component 1104 of the input optical pulse 1102, a polarization controller 1105, an optical fiber 1106 with a particularly large polarization mode dispersion such as a polarization maintaining fiber, a TE or TM polarization component 1107, a TM or TE polarization component 1108, an optical coupler 1109, a photodetector 1110, an electric band-pass filter 1111, a control system 1112 of the polarization controller 1105, an output optical fiber 1113, a waveform reshaped optical pulse 1116, a TE or TM polarization component 1114 of the optical pulse 1116, and a TM or TE polarization component 1115 of the optical pulse 1116 (see, George Ishikawa, Hiroki Ooi, and Yuichi Akiyama, APCC/OECC ""99, pp. 424-428).
The configuration of FIG. 2 uses a scheme for compensating the polarization mode dispersion by adjusting the polarization state of the input optical pulse 1102 by the polarization controller 1105 such that the delayed polarization component 1103 will be entered into a fast propagation direction of the optical fiber 1106 while the advancing polarization component 1104 will be entered into a slow propagation direction of the optical fiber 1106. As the optical fiber 1106, one with a particularly large polarization mode dispersion such as the polarization maintaining fiber is used. The polarization maintaining fiber has the polarization dispersion of about 1 ps per 1 m, for example.
According to the configuration of FIG. 2 disclosed in the above mentioned reference, a part of the optical signal is split at the optical coupler 1109 and detected at the photodetector 1110, and an electric signal obtained by the photoelectric conversion of the detected light at the photodetector 1110 is sent to the control system 1112 through the electric band-pass filter 1111 with a bandwidth equal to one half of the transmission speed. The control system 1112 controls the polarization controller 1105 to maximize the intensity of the electric signal (i.e., the intensity of the detected light), so as to minimize the polarization mode dispersion, i.e., to minimize a difference between the differential group delays of the polarization component 1114 and the polarization component 1115, such that the waveform reshaped optical pulse 1116 can be obtained.
However, the conventional art shown in FIG. 2 has the following drawbacks. The first drawback is a limitation on the bit rate of the optical signals. The configuration of FIG. 2 requires the electric band-pass filter 1111 with a bandwidth equal to exactly one half of the bit rate, so that the bit rate cannot be changed. The second drawback is that, when xe2x80x9c10xe2x80x9d codes appear consecutively as in xe2x80x9c10101010 . . . xe2x80x9d, for example, the higher harmonic component at one half of the bit rate increases so that there is a problem of affecting the electric feedback. The third drawback is that it requires the photodetector 1110 with a speed equal to one half of the bit rate so that there is a problem of making the photodetector 1110 expensive.
It is therefore an object of the present invention to provide a polarization mode dispersion compensating device which is capable of changing the bit rate, which does not affect the feedback system even when xe2x80x9c10xe2x80x9d codes appear consecutively, and which can be formed by using a low speed photodetector.
It is another object of the present invention to provide a polarization mode dispersion compensating device which is capable of effectively compensating the polarization mode dispersion even when a difference between the optical powers of the TE polarization component and the TM polarization component is large.
According to one aspect of the present invention there is provided a polarization mode dispersion compensating device, comprising: a polarization mode dispersion equalizer configured to receive an input optical signal propagated through an optical fiber, and output a polarization mode dispersion compensated optical signal by compensating a polarization mode dispersion of the input optical signal such that a difference between transmission delays of a TE polarization component and a TM polarization component of the input optical signal becomes minimum; a polarization component splitting unit configured to receive the polarization mode dispersion compensated optical signal outputted from the polarization mode dispersion equalizer, and split a part of the polarization mode dispersion compensated optical signal into the TE polarization component and the TM polarization component; an optical XOR circuit configured to receive the TE polarization component and the TM polarization component split by the polarization component splitting unit separately at two input ports through an identical optical path length, and output a logical operation result of an optical XOR operation on the TE polarization component and the TM polarization component entered at the two input ports; and a control system configured to control compensation by the polarization mode dispersion equalizer such that the logical operation result outputted by the optical XOR circuit becomes xe2x80x9c0xe2x80x9d.
According to another aspect of the present invention there is provided a polarization mode dispersion compensating method, comprising the steps of: compensating a polarization mode dispersion of an input optical signal propagated through an optical fiber, by controlling a polarization state of the input optical signal by a polarization controller and compensating a polarization mode dispersion of a polarization state controlled optical signal by having the polarization state controlled optical signal propagated through a polarization mode dispersion controlling optical fiber, such that a difference between transmission delays of a TE polarization component and a TM polarization component of the input optical signal becomes minimum; splitting a part of the polarization mode dispersion compensated optical signal into the TE polarization component and the TM polarization component; adjusting optical powers of the TE polarization component and the TM polarization component to be equal, when a polarization state of the input optical signal is not in a special state in which a difference between the optical powers of the TE polarization component and the TM polarization component is less than or equal to a prescribed value; carrying out an optical XOR operation on the TE polarization component and the TM polarization component after the adjusting step; and controlling a control of the polarization state by the polarization controller such that a logical operation result of the optical XOR operation becomes xe2x80x9c0xe2x80x9d.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.